1. Field of the Invention
The present invention relates to a horizontal-field-drive liquid crystal display, and particularly to a horizontal-field-drive liquid crystal display device that maintains a high aperture ratio while suppressing the occurrence of display defects such as vertical crosstalk and stepping non-uniformity.
2. Description of the Related Art
Liquid crystal displays of the prior art are typically of a type in which an image is displayed on a panel by causing an electric field to act in the direction perpendicular to the substrate surface, causing change in the alignment of the director (molecular axis) of the liquid crystal molecules, and thus controlling the transmittivity of light (hereinbelow, this type is referred to as “vertical-field-drive”). Twisted Nematic (TN) mode is representative of this type. In liquid crystal displays of this vertical-field-drive type, the director is aligned perpendicular to the substrate surface when the field is being applied. As a result, the refractive index changes with the direction of viewing, thereby strengthening the viewing angle dependency and making a wide viewing angle difficult to obtain.
In contrast, recent years have seen research and development of horizontal-field-drive liquid crystal displays or IPS (In-Plane Switching) mode liquid crystal displays in which image display is realized through the control of the transmittivity of light by aligning the director of liquid crystal molecules parallel to the substrate surface, and causing an electric field to act in a direction parallel to the substrate surface to cause the director to rotate within a plane parallel to the substrate surface.
As an example of a typical horizontal-field-drive liquid crystal display (hereinbelow referred to as simply “IPS liquid crystal display” or merely, “liquid crystal display”), the construction of an IPS liquid crystal display described in Japanese Patent Laid-open No. 36058/95 is next described with reference to FIG. 1 and FIG. 2. FIG. 1 is a plan view showing the structure of one pixel of the IPS liquid crystal display, and FIG. 2 is a sectional view showing the layer structure of the IPS liquid crystal display at line V—V of FIG. 1.
Prior-art IPS liquid crystal display 10 has a plurality of pixels arranged in matrix form. As shown in FIG. 2, the display is provided with first transparent substrate (TFT substrate) 13, second transparent substrate (facing substrate) 15, and liquid crystal component layer 44 sealed between first transparent substrate 13 and second transparent substrate 15.
First transparent substrate 13 is provided with first glass substrate 12 on which electrodes (16, 20, 22) and switching structures (30, 32) are formed, and first alignment layer 28 is formed on the highest layer. Second transparent substrate (facing substrate) 15 is provided with second glass substrate 14, light-shielding layer 36, and second alignment layer 42, which are formed successively on second glass substrate 14. Second transparent substrate 15 is arranged such that second alignment layer 42 is parallel to and confronts first alignment layer 28 of first transparent substrate 13.
Further, as shown in FIG. 1 and FIG. 2, electrodes (16, 20, 22) are made up of, for each pixel, two common electrodes 16A and 16B, first insulating film 18 formed over common electrodes 16, signal line (drain line) 20, and pixel electrode 22. The two common electrodes 16A and 16B extend over first glass substrate 12 separated from and parallel to each other. Signal line 20 extends parallel to common electrodes 16 and is positioned over first insulating film 18 and between common electrode 16B of one pixel and common electrode 16A of the neighboring pixel. Pixel electrode 22, similar to signal line 20, extends parallel to common electrodes 16 and is positioned over first insulating film 18 and between common electrodes 16A and 16B. Common electrodes 16A and 16B are each connected to common line 24.
First alignment layer 28 is stacked over signal line 20 and pixel electrode 22 with second insulating film 26 interposed. Pixel electrodes 22 and common electrodes 16 are actually alternately arranged so as to form a plurality of pairs.
The switching mechanism (30 and 32) is made up of thin-film transistor 32 and scan line 30, which drives thin-film transistor 32.
The gate electrode of thin-film transistor 32 is connected to scan line 30, the drain electrode is connected to signal line 20, and the source electrode is connected to pixel electrode 22.
Black matrix 36 for shielding light which has an aperture region (hereinbelow referred to as aperture 34) on the pixel is formed on the surface of second glass substrate 14 that faces first transparent substrate 13, as shown in FIG. 2, and color filter 38 is formed over aperture 34 and over black matrix 36 around the periphery of aperture 34. Aperture 34 is opened in black matrix 36 as a rectangle demarcated by common line 24, scan line 30, and two common electrodes 16, as shown in FIG. 1. Light advances through aperture 34 from the side of first transparent substrate 13 and toward the side of second transparent substrate 15 to realize image display.
Second alignment layer 42 is formed on color filter 38 of second glass substrate 14 with planarization film 40 interposed. The initial direction of orientation of the liquid crystal molecules of second alignment layer 42 is the same direction as that of first alignment layer 28.
Liquid crystal component layer 44 is accommodated and sealed between first alignment layer 26 and second alignment layer 42.
As shown in FIG. 1, liquid crystal molecules are oriented by the aligning function of first alignment layer 26 and second alignment layer 28 such that their director forms any angle θ that is not orthogonal or parallel to the longitudinal direction of pixel electrode 22.
In addition, polarizing sheet (not shown) are provided on the outer sides of first glass substrate 12 and second glass substrate 14. For both polarizing layers, the polarizer absorption axis, which is the direction of the polarizer that absorbs light, is parallel to the rubbing angle, and the analyzer absorption axis, which is direction of an analyzer that absorbs the light, is arranged orthogonal to the rubbing angle, as shown in FIG. 1.
The operation of above-described liquid crystal display 10 of the prior art is next described. Thin-film transistor 32 switches the electrode structure ON and OFF in accordance with ON/OFF signals received from scan line 30. When thin-film transistor 32 is ON, charge flows from signal line 20 to pixel electrode 22. When thin-film transistor 32 is OFF, pixel electrode 22 holds the charge and maintains a particular set potential. In contrast, a constant fixed direct-current voltage is applied to common electrodes 16.
A horizontal electric field is produced within liquid crystal component layer 44 in the direction parallel to first glass substrate 12 due to the difference in electric potential between pixel electrode 22 and common electrodes 16, and the liquid crystal molecules on pixel electrode 22 move. The potential difference between pixel electrode 22 and common electrodes 16 is held until writing of a signal of reversed polarity, and the display written to pixel electrode 22 is therefore displayed by way of aperture 34 of black matrix 36.
The direction of the electric field of common electrodes 16 and the regions of the liquid crystal component layer in the vicinity of common electrodes 16, and accordingly, the regions of the liquid crystal component layer alongside signal lines 20, is not parallel to first glass substrate 12, but rather, is close to perpendicular to glass substrate 12. The optical transmittance therefore cannot be controlled as desired, the luminance is disturbed, and light that passes through this region must be blocked by black matrix 36.
The above-described IPS liquid crystal display of the prior art has the following problems.
First, these liquid crystal displays are prone to a problem called vertical crosstalk in which, when displaying a white window in a black display screen as shown in FIG. 3A, the black display regions above and below the window of the display screen appear faintly white compared to other black display areas when the display screen is viewed from a side.
Second, there is the problem that, in the case of forming the electrodes of first and second transparent substrates by patterning by stepping projection alignment by a stepper, stepping non-uniformity is produced along the borders of the stepping projection alignment, as shown in FIG. 3B.
Regarding the cause of the first problem, as described hereinabove, the proper control of light transmittivity is impossible, i.e., luminance cannot be controlled, in the regions adjacent to signal lines 20, i.e., the regions between signal line 20 and common electrodes 16 (the regions indicated by “g” in FIG. 4, and referred to hereinbelow as “region g”). Moreover, the luminance of these g regions varies with fluctuation in the electric potential of the signal line with respect to the common electrodes and varies with fluctuation in the distance between signal line and common electrodes.
The disadvantage of the vertical crosstalk shown in FIG. 3A arises from the change in electric potential of the signal lines with respect to common electrodes. Signal potential is applied to the signal lines of pixels in which there is white display, and the potential in these signal lines is the average voltage of signal lines and therefore higher than average voltage of the signal lines of pixels in which there is no white display. Luminance thus varies under the influence of this difference in the portions above and below a window when viewed from an angle.
As can be understood from FIG. 4, region g is located exactly behind black matrix 36 and cannot be seen when a user views the display screen from directly in front of the screen, and vertical crosstalk therefore does not occur in the display screen. In contrast, region g is visible when the user views the display screen from an angle, as shown in FIG. 4, and vertical crosstalk therefore occurs in the display screen.
Regarding the cause of the second problem, the disadvantage of the stepping non-uniformity shown in FIG. 3B arises due to the variation in distance between signal lines and common electrodes. The distance between signal lines and common electrodes varies due to aberration in the overlay of the layer in which common electrodes are formed and the layer in which signal lines and pixel electrodes are formed during stepping projection alignment by a stepper. This variation in distance results in variation in the luminance of region g, which in turn causes stepping non-uniformity when the screen is viewed from an angle.
Two methods have been considered in the prior art for solving the above-described disadvantages of displays:    (1) Enlarging the dimensions of the black matrix such that region g cannot be seen even when the display screen is viewed from an angle; and    (2) overlaying the common electrodes and signal line such that region g does not exist. However, these two methods have the following problems: First, as shown in FIG. 5A, making the dimensions of black matrix 36 on the facing substrate-side equal to or greater than the distance between the outer edges of common electrodes 16A and 16B on the first transparent substrate side in the above-described method (1) hides region g from view even when the display screen is viewed from an angle.
In a case in which the black matrix extends beyond the common electrodes, however, the area of the aperture decreases as shown in FIG. 5A and the aperture ratio decreases. In addition, if a construction is adopted in which the outer edges of black matrix 36 are aligned with the outer edges of common electrodes 16, change or fluctuation in processing conditions causes a divergence in the positions of overlay of first transparent substrate and second transparent substrate. This brings about a corresponding positional shift between black matrix 36 and common electrodes 16 as shown in FIG. 5B, whereby the area of the aperture decreases and the aperture ratio falls.
In the above-described method (2) in which the common electrodes and signal lines are overlapped so as to eliminate region g, parasitic capacitance 46 increases between common electrodes 16 and signal lines 20 as shown in FIG. 6A. This capacitance effect weakens the signal waveform or increases transfer delay and prevents the accurate writing of potential to each pixel.
In addition, the presence of any holes or voids in interlayer insulation film 18 in the portions of overlap of common electrodes 16 and signal lines 20 as shown in FIG. 6B gives rise to shorts and the increased probability of a defective display device that is incapable of normal display.
As described hereinabove, IPS liquid crystal displays of the prior art were not easily amenable to preventing display defects such as vertical crosstalk and stepping non-uniformity when the display screen is viewed at an angle from the side horizontally.